Wire strikes are a potentially fatal risk facing all rotor and fixed wing aircraft, particularly those engaged in low flying activities such as nap of the earth (NOE) military flying, crop spraying, hunting, sightseeing surveillance and the like. While maintaining vigilant observation and researching potential flight path hazards reduce the likelihood of wire strike, wires and lines remains extremely difficult to detect from a cockpit. Even if detected, accurately discerning the distance from the craft to a power line can be very difficult. This is further compounded in the case of thinner less visible lines such as communication lines. Research has shown that in over 40% of wire-strike accidents, the pilot was aware of the existence of the wire. Moreover, contrary to popular misconceptions, the majority of wire strikes are incurred by experienced pilots in clear weather.
It is thus not possible to rely on pilot awareness to avoid wire strikes and so a form of on-board defence system may provide a potentially life saving feature. Several systems have been trialled including the mechanical wire cutters described in the following patents:
GB 2,075,940: WO 99/38770: U.S. Pat. No. 4,826,103; 5,286,170 and 5,415,364 each describe means of mechanically severing a wire impacting the aircraft. The mechanism employed typically involves some form of guide elements configured to urge the wire into a set of jaws for severing, either by fixed sharpened surfaces or a guillotine powered by explosive actuators. These systems posses several shortcomings including;                a potential hazard to ground crew from the guide elements used to trap the wire in the cutting jaws which typically project forwards from the aircraft fuselage/wings;        increased windage, cost and unwieldiness,        increasing the visible and electromagnetic signature of the craft detrimental to stealthy military operations.        the ‘capture’ area of the severing jaws is relatively narrow leaving other areas of the craft vulnerable to impact damage;        reliance on moving actuators to sever the wire;        only provides protection against frontal impacts in forward flight; and        may be dependant on the forward speed of the craft and the sharpness of the cutting surfaces.        
A key disadvantage of fixed guillotine-type wire cutters is their dependence on the speed of impact to provide the necessary energy to force the wire between the cutting jaws to be severed. Consequently, a slow flying aircraft/helicopter may not provide sufficient impetus to instantaneously sever the wire, whilst the deceleration due to the impact may reduce the craft's airspeed to the point of stalling and thus still precipitating a crash.
This effect is compounded by the degree of slack common in most suspended wires and lines. An aircraft may be sufficiently decelerated by taking up the slack in the wire to stall the aircraft and/or fail to force the wire through the cutting laws with insufficient energy for severing.
It is thus desirable for a wire-strike defence system to operate without complete dependence on the airspeed of the aircraft.
U.S. Pat. No. 4,826,103 discloses a means of addressing the aforesaid disadvantages in static mechanical wire cutters by way of an automatic-firing, powered guillotine. Whilst this provides a more instantaneous response to wire-strike, the system is complex and depends on several moving parts and interacting components with an attendant reliability and cost liability.
U.S. Pat. No. 4,407,467 implements a different cutting technique to the above, whereby a forwardly extending inclined boom or ‘probe’ is fitted with a plurality of explosively-powered cutting means. An impacted wire slides along the probe until encountering a cutting means which triggers an explosion. It the cable remains un-severed, the wire continue along the probe to the next cutting means triggering a second explosion and so on. Nevertheless, the system disclosed still suffers from several of the above-described drawbacks and in particular presents an unwieldy structure for attachment to an aircraft together with the clear hazards associated with the presence of multiple explosive devices located adjacent the aircraft.
Consequently, there is a need for a robust compact wire-strike defence system that may be placed unobtrusively on any chosen portion of the aircraft which preferably operates independently of the craft's airspeed.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.